1. Field of the Invention
The present invention relates to a recording apparatus having a heat generating element which is an electro-thermal transducer and a recording apparatus using the same.
2. Related Background Art
An ink jet recording method permits a high speed printing with a negligibly low noise generated during printing and permits recording by fixing on a plain paper without special treatment and hence it has been recently attracting notice.
Among others, an ink jet recording method disclosed in Japanese Patent Application Laid-Open No. 54-51837 or German Patent Laid-open (DOLS) 2843064 has a feature different from those of other ink jet recording method in that it acts a thermal energy to liquid to obtain a motive force to discharge liquid droplets. Namely, in the recording method disclosed in the above-mentioned case, the liquid which received the action of the thermal energy causes a status change with an abrupt volume increase and ink is discharged from an orifice at an end of an ink jet head to form flying liquid droplets. Those liquid droplets are deposited on a recording sheet to form a record.
In particular, the ink jet recording method disclosed in DOLS 2843064 is effectively applied to a so-called drop-on-demand recording method. Further, since a multi-orifice ink jet recording head of a high density full line type can be readily implemented, a high resolution and high quality image can be attained at a high speed.
The ink jet recording head of an apparatus used for this method includes a print head substrate which comprises a liquid discharge unit including an orifice provided to discharge liquid and a liquid flow path coupled to the orifice and having a thermal action unit by which a thermal energy for discharging liquid droplets is acted to the liquid, and an electro-thermal converter (heat generating element) as means for generating the thermal energy.
Recently, as the print head substrate of the above type, a substrate comprising a plurality of heat generating resistors arranged in a line, drivers provided one for each of the heat generating resistors for driving the heat generating resistors in accordance with image data, a shift register of the same number of bits as the number of the heat generating resistors for parallelly outputting the serially inputted image data to the respective drivers, and a latch circuit for temporarily storing the data outputted from the shift register, all mounted in one substrate, has been developed.
A circuit configuration of such a prior art print head substrate 400 is shown in FIG. 4.
Referring to FIG. 4, numeral 401 denotes heat generating elements arranged in a line, numeral 402 denotes a power transistor which functions as a driver, numeral 403 denotes a latch circuit and numeral 404 denotes a shift register. Numeral 405 denotes a terminal to which a clock signal for shifting data into the shift register 404 is applied and numeral 406 denotes a terminal to which serial image data is applied. Numeral 407 denotes an input terminal of a latch signal, numeral 408 denotes a heat pulse input terminal for externally controlling an on time of the power transistor 402, numeral 409 denotes a logic power supply terminal and numeral 410 denotes a ground terminal. Numeral 411 denotes a heat generating resistor driving power supply (VH) input terminal.
In a printer apparatus having a head including the print head substrate of the above configuration, the serial image data is serially inputted from the input terminal 406 to the shift register 404. The image data loaded to the shift register 404 is latched in the latch circuit 403 by the latch signal applied from the terminal 407. When a pulse is applied from the heat pulse input terminal 408, the power transistors 402 corresponding to the "1" image data are turned on. Thus, the corresponding heat generating resistors 401 are energized and the liquid (ink) in the liquid flow paths of the energized heat generating resistors 401 are heated and the ink is discharged from the discharge ports so that the printing is made.
Considering the energy necessary to generate bubbles in the liquid contacting to the heat generating resistor, if a heat dissipation condition is constant, the energy is a product of an energy required per unit area of the heat generating resistor and an area of the heat generating resistor. Thus, a voltage across the heat generating resistor, a current flowing through the heat generating resistor and a time (a pulse width) may be set to meet the above energy condition. In an actual use, the voltage may be set to be substantially constant by the power supply of the printer apparatus but as for the current, the resistance of the heat generating resistor may vary from rot to rot and from substrate to substrate due to variation of film thickness of the heat generating resistor during the manufacturing process of the substrate. Accordingly, when an applied pulse width is constant and the resistance of the heat generating resistor significantly is larger than the setting, the current is small and the applied energy is insufficient so that bubbles are not generated in the ink. On the other hand, when the resistance of the heat generating resistor is smaller and the current flowing through the heat generating resistor is larger than the setting, an excessive energy is applied and the heat generating resistor is baked and the lifetime of the heat generating resistor is shortened. To solve this problem, the resistance of the heat generating resistor 401 may be continuously monitored by a sensor 414 and the power supply voltage or the applied pulse width may be changed in accordance with the resistance so that a constant energy is applied.
Now, considering the amount of discharge of the liquid droplets, the amount of discharge primarily relates to a volume of bubbles of the ink. Since the bubble volume of the ink changes with a temperature of the heat generating resistor and a surrounding temperature, the temperature of the heat generating resistor and the surrounding temperature may be adjusted by a pulse width and a timing of a pulse (pre-heat pulse) of an energy of not discharging the ink applied prior to the application of the heat pulse for discharging so that a constant amount of droplets is discharged to maintain a print quality.
In accordance with the prior art, the compensation of the variation of the resistance of the heat generating resistor 401 and the control of the substrate temperature are conducted by feeding back the signals from the sensors 414 and 415 for monitoring the respective values to change the width of the heat pulse applied to the heat generating resistor 401, the width of the pre-heat pulse and the timing thereof under the control of the printer apparatus to output the heat signal. However, the amount of discharge of the ink varies from nozzle to nozzle due to the variation of the area of the orifice opening during the manufacture and the variation of a film thickness of a protection film for the heat generating resistor 401, and it leads to an irregular density and a stripe of the print and hence the control of the amount of discharge for each nozzle or for every several nozzles is required. Further, as the number of nozzles of the ink jet head increases, when a plurality of print head substrates are serially connected to form a multi-nozzle ink jet head, a resistance of the heat generating resistor varies from substrate to substrate and the energies applied to the respective substrates must be made substantially equal by changing the heat pulse for discharging the ink for each substrate. When the head is constructed by a plurality of substrates, a difference of print density between substrates becomes prominent in addition to the area of the orifice and the correction of the amount of discharge for each nozzle in the substrate is more important than for the single substrate.